Cellular Na+/H+ exchanger (NHE) activity is elevated in type 1 diabetic patients with nephropathy and patients with essential hypertension. The characteristics of this NHE phenotype in hypertension (raised Vmax and a lowered Hill coefficient) are preserved in Epstein-Barr virus-transformed lymphoblasts from hypertensive patients. In this study, we have determined NHE kinetics in cultured lymphoblasts from diabetic patients with and without nephropathy, with nondiabetic controls, using fluorometry with the pH indicator 2,7'-bis-(carboxyethyl)-5,6-carboxyfluorescein and estimation of NHE isoform 1 (NHE-1) density with specific polyclonal antibodies. The Vmax of NHE was elevated significantly, and the Hill coefficient for internal H+ binding was lowered in cells from patients with diabetic nephropathy compared with both normal controls and normoalbuminuric diabetic patients. NHE-1 density as measured by Western blotting was similar in all groups. The turnover number of NHE-1 was thus elevated in cells from nephropathy patients. This phenotype in cells from diabetic nephropathy patients resembles that in essential hypertension and suggests that such patients may have a predisposition to hypertension. Moreover, as these changes persist in cultured lymphoblasts in vitro, these cells should provide a cell culture model to further define the basic mechanisms leading to NHE activation in diabetic nephropathy.